The invention relates to a process for preparing tetrahydrofuran and/or butane-1,4-diol and/or gamma-butyrolactone, especially to a process for preparing tetrahydrofuran (THF) from succinic acid which has been obtained by conversion of biomass, by conversion of the succinic acid to succinic anhydride and hydrogenation of the succinic anhydride, with removal of troublesome secondary components.
The preparation of THF by hydrogenation of carboxylic acid derivatives such as maleic anhydride, maleic acid, maleic esters, succinic anhydride, succinic acid and succinic diesters is known per se. For instance, DE 100 61 556 A1 describes the hydrogenation of dicarboxylic acids and derivatives thereof over Cu catalysts in the gas phase. The emphasis is the hydrogenation of maleic anhydride prepared by gas phase oxidation, for example, of butane. WO 2003 006 446 gives a similar description, the emphasis here being on the hydrogenation of the diesters. None of the documents mentions how THF is prepared proceeding from succinic acid prepared by fermentation.
EP 2 476 674 A2 describes how cyclic compounds (lactones and ethers) are prepared proceeding from C4-C6 carboxylic acids or esters thereof. There is explicit mention of the use of biomass-based acids or esters produced therefrom. Preference is given to using a catalyst which is neutral, in order that no by-products that result from dehydration are generated. There is no mention of the use of succinic anhydride. Nor is there any teaching as to how impurities present in the starting materials are removed or the influence thereof is limited to a minimum.
The preparation of succinic acid from biomass is described, for example, in WO 2010/092155 A1. In addition, this WO also describes the further processing of succinic acid or diesters thereof to obtain THF, butanediol and/or gamma-butyrolactone, the esters being obtained, for example, by reactive distillation esterification of diammonium succinate. Otherwise, salts of succinic acid are converted to free succinic acid by acidic ion exchangers which are then regenerated, for example, with HCl. The purification of this succinic acid thus obtained is then effected by concentration and crystallization. In example 9, the purity of the succinic acid is stated as 99.8%. There is no description of what impurities are present and how these may then effect the hydrogenations. Nor is there any mention of succinic anhydride.
The preparation of succinic anhydride from succinic acid is known per se. DE-A-1 141 282 describes the preparation of succinic anhydride by feeding liquid succinic acid into a column, wherein water is distilled off overhead and the anhydride is obtained via the bottom with a purity of 95%-97%. Nothing is said about the impurities. According to FR 1 386 278, succinic acid is dewatered by distillation, leaving the anhydride in the residue. According to Org. Lett. 2011, 13, 892, succinic acid is formed in a homogeneously catalyzed process with the catalyst as high boiler; the purification is effected by precipitation of the anhydride and filtration of the reaction mixture.
JP 2003 113 3171 A describes the purification of succinic anhydride by distillation, wherein the distillation of the crude succinic acid to avoid discoloration of the product the bottom temperature at reduced pressure is between 125 and 200° C. Only dilactones are described as impurities to be avoided. There is no mention of the preparation of succinic anhydride by means of succinic acid obtained from fermentative processes. In addition, no effect on downstream hydrogenation steps is detailed; instead, only discoloration of polyesters prepared with contaminated succinic anhydride is mentioned.
In contrast to products which are prepared by conventional chemical reactions, the peculiarity of products which are obtained from biomass lies in the disproportionately higher number of secondary components which can disrupt downstream applications, particularly when polymers having long chain lengths are to be formed, where monofunctional groups disrupt the formation of chains. In this present case, a main use of THF is the preparation of polytetrahydrofuran. Moreover, secondary components can have a disruptive effect on catalysts in terms of selectivity and yield of the process, but in particular have an adverse effect on the lifetime of the catalysts. Examples of these disruptive secondary components which can be harmful even in amounts below 1 ppm by weight comprise the elements N, P, S, As, Sb, Bi, Sn and halogens such as Cl, Br and I.
N-comprising compounds, particularly when they have basic properties, can occupy acidic centers on catalysts and hence destroy desirable properties. More particularly, the nitrogen-comprising compounds are harmful when they pass through a hydrogenation step, since it is only this that causes many of them to become basic compounds. This can then hinder the hydrogenation step or downstream processes. Here, the polymerization of THF is again adversely affected, since it is conducted in the presence of acidic catalysts. Especially in the case of hydrogenation of succinic anhydride in the presence of N-containing catalysts, especially of ammonia or those that can release it, it is easy for pyrrolidine to form, which hinders the hydrogenation process and, because of its boiling point being similar to that of THF, is separable therefrom only with difficulty, and disrupts the polymerization at a later stage.
Compounds comprising P, S, As, Sb, Bi, Sn or halogens such as Cl, Br and I are undesirable, since some are not just toxic to the environment but can also poison hydrogenation catalysts. Many of these compounds are volatile, and so they can partly withstand distillative purifying processes or get into the stream to be hydrogenated in gas phase processes.
In fermentative processes for preparing succinic acid, acids formed by way of example are not just succinic acid but also a number of other acids such as formic acid, acetic acid, propionic acid and butyric acid. Because of their acid strength, these are capable of damaging catalysts.